Resistance spot welding is a process by which faying surfaces are joined in one or more spots by the heat generated by the resistance to the flow of electric current through workpieces that are held together under force by electrodes. The contacting surfaces in the region of current concentration are heated by a short-time pulse of low-voltage, high-amperage current to form a fused nugget of weld metal. When the flow of current ceases, the electrode force is maintained while the weld rapidly cools and solidifies. The electrodes are retracted after each weld, which usually is completed in a fraction of a second.
Resistance-welding applications have grown tremendously since 1933, when the first all-steel welded automobiles were introduced. The popularity of resistance spot welding is due to the fact that this method produces welds rapidly and lends itself to automation and inclusion in high-volume, rapid production assembly lines with other fabricating operations. These advantages are offset by a serious deficiency in spot-welding, namely, the inability to control the process satisfactorily in order to produce consistently good welds. The reason for this deficiency in good control is that there are many variables which must be controlled or which vary from weld to weld, such as voltage, current, pressure, heat loss, shunting, electrode wear, and thickness and composition of the workpiece material. Some of these variables are difficult or practically impossible to control.
Throughout the years attempts have been made to automatically control resistance spot welding processes by regulating the electrical energy and, thus, the resulting heat. Current sensors have been used to provide feedback in order to maintain a constant welding current. Voltage regulators have been incorporated to compensate for line voltage variations or impedance changes. However, all these feedback systems are based on controlling process conditions according to set reference levels that, at best, have been determined empirically. There is no feedback from the actual weld process itself which could be used to control the variables in order to produce a good weld.
The vast majority of spot-welding machines are governed by timers which control four basic steps:
1. close electrodes and apply force (squeeze time);
2. initiate and maintain current (weld time);
3. turn off welding current and maintain electrode force until weld nugget solidifies (hold time);
4. open electrodes (off time).
The times required by these steps are empirically adjusted for optimum performance and remain fixed throughout subsequent spot-welding operations. It is generally assumed that the weld time is sufficiently long to bring the metal into the molten state. This is not always the case. With increasing wear of the electrodes the time needed to bring about the molten state increases and may at times be longer than the pre-set "weld time." The indentation in this case does not reach the required percentage, and a poor weld will be the result.
The present invention, as set forth hereinafter, is based on the fact that there is, indeed, one necessary condition for producing a weld--the metal must reach a molten state. If the metals to be welded do not reach the temperature required to become molten, an insufficient weld or no weld at all will result. The detection of the molten phase, which is dependent on the welding process and not on fixed parameters, is used in this invention to control the welding variables. It has been shown through measurements that as soon as the molten state is reached, the electrodes, which are being forced against the workpiece, begin to move into the metal and towards each other. This electrode movement (indentation), although generally only a few percent of the sheet metal thickness, is thus an indication of the molten phase.
The graphs in FIG. 1 of the accompanying drawings are reproduced from RWMA Resistance Welding Manual, 3rd Ed., Vol. 1, p. 122. They show the relationship between electrode indentation, breaking load (tensile-shear) of the welded joint and the diameter of the nugget for welds made at various currents. According to that reference the welds were made in annealed low carbon steel, 0.029 in. thick, using a type A (pointed) electrode with a 1/4 in.-diameter tip, an electrode pressure of 15,000 psi (a force of about 735 lbs.) and a weld time of 6 cycles. The graphs show that at the optimum current value (13,500 amp.) the diameter of the nugget was nearly the same as the diameter of the electrode tip (1/4 in.). Increasing the current above 13,500 amp did not significantly increase the nugget diameter, but caused a marked increase in electrode indentation. Tensile-shear breaking load increased rapidly until the optimum current was reached, but decreased slightly when the current was increased to slightly above 14,000 amp. Indentation increased from about 2% of the sheet thickness at a welding current of 13,500 amp to about 10% at a welding current slightly above 14,000 amp.
It is apparent from the graphs in FIG. 1 that the detection of melting and subsequent movement of electrode (indentation) is potentially a good way of determining the state of the weld. Attempts which have been made in the past to use detection of electrode movement to control the welding process have, however, been unsuccessful. The reason for this is the difficulty of measuring in an accurate and repeatable way the small distances involved in the travel of spot-welding electrodes, which are of the order of 0.001 inch.